An assembled battery (battery system) configured by connecting a plurality of secondary single cells in series is used for a hybrid electric vehicle (HEV), an electric vehicle (EV), or the like to secure a desired high voltage. Such an assembled battery uses a control IC which monitors the states of the single cells and controls the states of charge and discharge, and a battery controller which controls the control IC to manage each single cell (see PTL 1).
In the battery system of PTL 1, four single cells constitute one battery cell group, and a control IC is connected to each battery cell group. A control IC at the highest level connected to a battery cell group on the highest potential side is activated in response to a start signal from the battery controller, and outputs, to a control IC one level below, the start signal at a voltage in accordance with the potential of a battery cell group corresponding to the control IC one level below. Such an operation is performed sequentially from a control IC at a higher level to a control IC at a lower level to activate all the control ICs.